A highly effective DC brush-less motor using a permanent magnet in a rotor has been employed for driving a compressor in an air-conditioning system of an automobile or as driving means of an electric vehicle, for example. Typically, the DC brush-less motor is driven by a drive control system shown in FIG. 6. The drive control system employs the PWM for a voltage controll in which a position of the rotor is detected by a voltage induced in the motor.
Specifically, the drive control system, i.e., inverter, includes a switching circuit 11 for converting a DC voltage supplied from a DC power source V.sub.DC into a modulated voltage (PWM voltage) of a series of pulses, a controller 13a for controlling the switching circuit 11 and a position detector 14 for detecting a position of the rotor 9 in the motor 7. Typically, the position detector 14 is an analogue circuit, arid the controller 13a is a semiconductor LSI chip.
The inverter so constructed has two operational modes; a start mode used for starting the motor and a normal mode used for driving the motor in a stable state after the completion of the start mode. Disadvantageously, a voltage induced at the start mode in a winding 8 of a stator is too small to be well detected, which makes it difficult to detect a position of the rotor 9. Due to this, the inverter is designed so that, at the start mode, the controller 13a controls the switching circuit 11 irrespective of the position of the rotor 9 for varying the PWM voltage applied to the motor 7. Subsequently, once a rotational speed of the motor 7 has been increased to a predetermined speed, the operational mode is changed from the start mode to the normal mode.
In the normal mode, the position detector 14 detects the position of the rotor 9 from the voltage induced in the winding 8 of the stator. Based upon tile detected position of the rotor 9, the controller 13a controls the switching operation of the switching circuit 11 for controlling the PMW voltage. In this control, the PWM voltage is determined by a carrier cycle (pulse recurrence time) and a duty ratio of the PWM voltage, both of which being controlled by the controller 13a.
Preferably and advantageously, when controlling the motor by using the inverter, the carrier cycle in the PMW voltage control is reduced to, for example, about several decades to several hundreds micron meters in order to minimize the undesirable sound noises generated from the motor. Each pulse width in the PWM voltage is determined using the carrier cycle and the duty ratio. As a result, in order to drive the motor in a lower speed, the duty ratio as well as the pulse width needs to be minimized.
FIG. 7A shows a waveform of voltage applied to the winding 8 of a certain phase in the stator with respect to the negative terminal of the DC voltage V.sub.DC. In the waveform, portions indicated by x and y correspond to the induced and applied voltages, respectively. FIG. 7B is an enlarged view of a part of the induced voltage, in which a series of voltage pulses are shown. As can be seen from FIG. 7B, each pulse of the induced voltage includes noises induced by an inductance of the winding 8 and floating capacities of switching elements A to F. Evidently, the noises provide the pulse with an adverse affect. This effect becomes problematic as the pulse width decreases.
To overcome this problem, the prior art position detector 14 is provided with an analogue filter circuit for removing the noises. The use of the analogue filter circuit, however, fails to ensure the stable performance of the position detector due to a frequency feature of the filter circuit and results in a time delay in the position detector.
Another method has been proposed for controlling the motor using a microcomputer in which a signal indicative of the induced voltage in the motor 7 is directly transmitted into the controller 13a, without passing through position detector 14. Hereinafter, this method will be referred to as "microcomputer based control". In this microcomputer based control, however, where the pulse at starting has a reduced width, the voltage variation due to the noise will make it difficult to precisely determine the induced voltage, i.e., pulse level.
Disadvantageously, incorporating the inverter controlled by the microcomputer into a compressor of the air-conditioning system of the automobile will increase the adverse affect, i.e., noise problem. One reason is that typically the inverter for the automotive air-conditioning system requires the reduced carrier cycle in PWM voltage control, compared with that used in another air-conditioning system for housing. For example, the air-conditioning system for electric vehicle requires a carrier cycle of about 100 .mu.s while the air-conditioning system for housing requires a carrier cycle of about 250 .mu.s. Another reason for that is that generally the automobile includes several sources that generate noises, such as driving inverter in the electric automobile and spark plugs in an internal combustion engine automobile.
In view of this, in the drive system using DC brush-less and sensor-less motor controlled by the microcomputer, a technique capable of precisely detecting the induced voltage in the motor, even under the existence of noises, has long been expected especially for the electric automobile and hybrid type electric automobile that employ the drive control system with the DC brush-less and sensor-less motor.